Agree with Wyo Traveler, you are not going to get great (relative) mileage with either a 5er or TT. On the other hand, when I sold my 29'TT and bought a 35' 5er, I experienced about a 1 to 2 mpg increase with the same tow vehicle.
2011 Ford 6.7. Average town-hiway mileage 16.7. Freeway about 20 at 70mph. Towing 18500 trailer, 10-12 depending on winds, terrain etc. Have owned all the Ford diesels except 6.4 and never had a problem with any.
Fifteen and 16 inch "ST" tires are the problem. Owning several travel trailers and two 5ers, I also had trouble with "ST" tires until I switched to LT tires. Specifically, since putting on Michelin XPS (16 inch) and Michelin XTA (17.5 inch) tires I have never had a tire problem. I simply do not believe the "ST" tires are up to the challenge. There are too many threads about "ST" tire failures to believe that they are quality.